1. Field of the Invention
The present invention relates to a DC/DC voltage converter and, more particularly, to a light loading control circuit for a buck-boost voltage converter.
2. Description of the Related Art
FIG. 1 is a circuit diagram showing a conventional buck-boost voltage converter. Referring to FIG. 1, the conventional buck-boost voltage converter includes a synchronous switching circuit 10, a buck-boost control circuit 11, a drive circuit 12, and a light loading control circuit 13.
The buck-boost control circuit 111 generates a voltage conversion control signal VCS in accordance with a feedback of an output voltage Vout. In response to the voltage conversion control signal VCS, the drive circuit 12 generates four drive signals D1 to D4 for driving switching units S1 to S4 of the synchronous switching circuit 10, respectively. Through appropriately turning ON/OFF the switching units S1 to S4 and adjusting a relationship between ON time and OFF time, an input voltage Vin is effectively regulated to an output voltage Vout regardless of the input voltage Vin being higher than, equal to, or lower than the output voltage Vout. In prior art are there a variety of types of the buck-boost control circuit 11, such as the circuits and methods disclosed in U.S. Pat. No. 6,166,527 and U.S. Pat. No. 6,788,033.
If only a relatively small loading current Iout is required by a load 50, then the conventional buck-boost voltage converter enters an operation of a light loading mode for enhancing the efficiency in the voltage conversion. In the operation of the light loading mode, the drive circuit 12 is under control of the light loading control circuit 13 instead of responding to the voltage conversion control signal VCS of the buck-boost control circuit 11. One example of the conventional light loading modes in the buck-boost voltage converter is disclosed in FIGS. 3 and 4 on page 10 of Datasheet LTC3440, entitled “Micropower Synchronous Buck-Boost DC/DC Converter,” published on year 2001 and by Linear Technology Corporation.
FIG. 2(A) is a timing chart showing an inductor current IL in the conventional light loading mode. Referring to FIG. 2(A), the conventional light loading mode has a rising phase and a falling phase. In the rising phase from time t1 to t2, the inductor current IL linearly increases from zero to a peak current Ipk. In the falling phase from time t2 to t3, the inductor current IL linearly decreases from the peak current Ipk to zero.
FIG. 2(B) is a schematic diagram showing an operation state of the synchronous switching circuit 10 in the rising phase. Referring to FIG. 2(B), the switching units S1 and S4 are turned ON and the switching units S2 and S3 are turned OFF, for coupling a first terminal La of an inductor L to the input voltage Vin and coupling a second terminal Lb of the inductor L to a ground potential. Therefore, an inductor current IL linearly increases with a rate of Vin/L.
FIG. 2(C) is a schematic diagram showing an operation state of the synchronous switching circuit 10 in the falling phase. Referring to FIG. 2(C), the switching units S1 and S4 are turned OFF and the switching units S2 and S3 are turned ON, for coupling the first terminal La to the ground potential and coupling the second terminal Lb to the output voltage Vout. Therefore, the inductor current IL linearly decreases with a rate of Vout/L.
For operating in the conventional light loading mode, an average of the loading current Iout is subjected to an upper limit, which may be calculated in accordance with the following equation (1):
                              I                                    out              ⁢              _              ⁢              ave                        ⁢                          (              max              )                                      =                              (                                          I                pk                            2                        )                    ⁢                      (                                          V                in                                                              V                  in                                +                                  V                  out                                                      )                                              (        1        )            
If the loading current Iout to be required becomes higher than the maximum average light loading current Iout—ave(max), then the conventional buck-boost voltage converter leaves the light loading mode and returns to the normal operation under the control of the buck-boost control circuit 11.
As described above, regardless of the input voltage Vin being higher than, equal to, or lower than the output voltage Vout, the buck-boost voltage converter regulates the input voltage Vin into the output voltage Vout. That is, the buck-boost voltage converter is applicable over a broad range of input voltages Vin. However, a variation of the input voltage Vin induces a change to the maximum average light loading current Iout—ave(max) of the conventional light loading mode. More specifically, a partial derivative of the maximum average light loading current Iout—ave(max) with respect to the input voltage Vin may be expressed in the following equation (2):
                                          (                          ∂                              ∂                                  V                  in                                                      )                    ⁢                      I                                          out                ⁢                _                ⁢                ave                            ⁢                              (                max                )                                                    =                              (                                          I                pk                            2                        )                    ⁢                      (                                          V                out                                                              (                                                            V                      in                                        +                                          V                      out                                                        )                                2                                      )                                              (        2        )            
Since the equation (2) is a function of the input voltage Vin, the light loading control circuit 13 commands the buck-boost voltage converter to leave the light loading mode in association with different maximum average light loading currents Iout—ave(max) with respect to different input voltages Vin.
Therefore, it is desirable to develop a light loading control circuit capable of stably controlling the activation and termination of the light loading mode of the buck-boost voltage converter over a broad range of input voltages Vin.